The invention concerns a selective composite diaphragm for electrolytic cells, which provides an improvement in the operating conditions of the cells.
Cells fitted with a porous diaphragm which separates the anode and cathode compartments have long been used in electrolysis. In such cells the phenomena of movement by diffusion, convection, percolation, and electro-migration cause uncontrollable transference of ions, leading to pollution of the effluent solutions. The concentrations of solutes in these solutions are limited and the yields are poor. Particularly in the case of electrolysis of brines for the preparation of chlorine and sodium, the sodium solutions obtained are polluted by salts and, in addition, are relatively dilute.
The use of ion-exchange membranes, with selectivity to the passage of cations or anions, in electrolytic cells has also been proposed. It is thus theoretically possible to avoid certain migrations of ions from one compartment of a cell to another by such membranes. In practice, the migration of ions which are subjected to the action of the magnetic field is only imperfectly controlled. In electrolysis of sodium chloride solutions with a cationic membrane, for example, as soon as the concentration of soda in the cathode compartment reaches a certain level, hydroxide ions pass to the anode compartment, thereby reducing the yield.
It is also difficult to find membranes which show satisfactory resistance to the chemical actions and mechanical stresses to which they are subjected and which, at the same time, show the desired selectivity in respect of the ions, at least in a concentrated medium.
It has been proposed to overcome these various difficulties by using cells with a plurality of compartments, particularly separated by partitions comprising at least one diaphragm and one membrane, each of such partitions separating two adjacent compartments. Apart from the obvious complexity of the cells, this results in a marked increase in the voltage requirement and in the obtaining of dilute solutions in most cases. Methods of electrolysis carried out in cells fitted with a diaphragm which is obtained by treating a diaphragm based on asbestos with an ion exchange resin which will react on asbestos have also been described. However, the concentrations of soda obtained are low and the solution obtained is polluted.
A new diaphragm has now been found which avoids the various disadvantages mentioned above. When, for example, it is used in electrolytic preparation of chlorine and soda, it enables pure, concentrated solutions of soda to be obtained with a high faradaic yield and also shows good resistance to corrosion and mechanical stresses.
It is, accordingly, an object of the present invention to provide a novel electrolytic cell and diaphragm which avoids the disadvantages of the prior art.
It is also an object of the present invention to provide an electrolytic cell having a diaphragm which enables one to provide substantially pure concentrated solutions of soda.
Other objects of the invention will be apparent to those skilled in the art from the present description.